1. Field of the Invention
The present invention relates to the fuel system of small two cycle crankcase scavenged spark ignition engines typically used for yard blowers and trimmers and similar equipment, to operate on LPG (liquified petroleum gas) such as propane, to eliminate air pollution, fuel mixing and providing single step starting, making these engines more user friendly. The present invention further relates to the substitution of a novel fuel system and carburetor assembly to replace the gasoline fuel tank and gas line conventionally used with such two cycle crankcase scavenged spark ignition engines.
2. Description of the Prior Art
A conventional two stroke engine is provided with a scavenging fan. At the start of the first stroke, the piston is in its highest position. When the compressed petrol-and-air over the piston is ignited, the latter is thrust downwards and, in so doing, releases the exhaust port. The burned gases in the cylinder, which are still under high pressure, can thus escape through this port. When the piston descends further, its upper edge releases the inlet port, which admits fresh petrol-and-air mixtures, delivered by a fan, into the cylinder, so that the remaining burned gases are flushed out. When the piston rises again on the second stroke, all the ports are closed off for a time, and during this period the petrol-and-air mixture is compressed, so that a fresh cycle can commence.
The crankcase-scavenged two-stroke engine has no scavenging fan. Instead, the crankcase is hermetically sealed, so that it can function as a pump in conjunction with the piston. When the piston ascends, a partial vacuum is produced in the crankcase, until the lower edge of the piston releases the inlet port and thus opens the way to the fresh petrol-and-air mixture into the crankcase. When the piston descends, the mixture in the crankcase is compressed a little so that, as soon as the top of the piston releases the transfer port and overflow duct which connects the crankcase to the cylinder, it can enter the cylinder. Meanwhile, what happens above the piston is the same as in the fan scavenged engine. The lubricating oil is mixed with the petrol or is, alternatively, supplied to the points of lubrication dropwise by small lubricating oil pumps. The oil which enters the crankcase is liable to be carried through the overflow duct and transfer port into the cylinder, where it passes through the exhaust port and into the exhaust system.
The two cycle crankcase scavenged spark ignition engines known as two cycle SI engines are predominantly used to power hand tools made for the garden industry and to a lesser extent for the construction and building industry. The power rating of these engines are from a few tenth of a horsepower to several horsepower. The two cycle SI engines dominate this field because they provide mobility at a light weight and a reasonable initial cost.
The fuel for these engines is a mixture of gasoline and lubricating oil in a predetermined ratio which must be mixed by the user. This is an unpleasant chore for the consumer, often resulting in improper mixture ratio. The fuel mixture provides fuel for the combustion process and the oil contained in it provides lubrication for the engine components. Because of the two cycle operating principle, part of the incoming mixture is discharged through the exhaust port as part of the scavenging process, leaving unburned hydrocarbons in the exhaust. Furthermore, much of the lubricating oil burns and contributes to smoke and unfavorable exhaust emission and odor.
The fuel system consists of a fuel tank attached to the engine and connected to a gasoline carburetor which is typically a membrane carburetor as manufactured by Walbro Co., Tillotson and several domestic and foreign manufacturers. This is a precision device consisting at least of thirty components and several minute holes. The engine crankcase pressure pulses are used to operate a small pulse pump to feed a metering chamber which dispenses fuel into the carburetor venturi as a function of air flow and throttle position. Providing starting fuel at cranking speed and controlling fuel at peak power is a considerable task which explains the need for complexity and required precision of this type of carburetor. As a consequence, the cost of the carburetor is high and is a considerable part of the total engine cost. Furthermore, it is one of the weak points of engine reliability and cause of hard starting. Starting of these engines is accomplished with a rope starter which has a spring rewind pulley and a ratchet that engages the crankshaft through pawls. When the engine starts, the pawls disengage due to the centrifugal force acting on them. The starting procedure involves several steps which must be performed in sequence. If it is done incorrectly, the engine will not start which is the most frequent consumer complaint. The typical starting sequence requires: to turn on the ignition, to close the choke, set the throttle to a starting position, briskly pull on the starter until the first pop is heard, readjust or open the choke, briskly pull on the starter until the engine starts, warm up the engine before loading. Starting a warm engine has its own problems such as vapor lock because of heat soak. There are also variations on the above procedure such as hand priming prior to starting.
Therefore, there is a significant need for an improved fuel system which operates on cleaner burning fuel such as liquid propane and which eliminates or significantly reduces the problems addressed above.